This invention relates to a method of forming honeycomb structures from joined cellular segments so as to improve the structure's thermal shock resistance, and in particular to a method of constructing a heat recovery wheel having improved thermal shock resistance and the wheel produced thereby.
Honeycomb structures are used in a variety of applications, such as catalytic reactors and heat recovery wheels for conditioning flowing fluids, primarily gases. Such structures consist primarily or entirely of a matrix having a plurality of apertures or hollow, open-ended cells which permit the passage of fluids through the structure. Because these structures are subjected to relatively severe thermal shock conditions during operation, they are commonly fabricated from ceramic or glass-ceramic materials having very low coefficients of thermal expansion. Other materials (e.g., glass, sintered metal, cermet or other ceramic base materials) could be employed as desired if they were suitable (e.g. sufficient strength, chemical resistance, refractoriness, thermal shock resistance, etc.) with the service conditions encountered.
Honeycombed structures are generally formed monolithically by the processes of extrusion of "wrapping" (the building up of corrugated layers). If sufficiently large, however, the structures must be built from joined cellular segments, themselves formed by either process. For example, for some time the Corning Glass Works has fabricated large heat recovery wheels (up to 70 inches (178 cm) in diameter) by joining cellular segments formed by the wrap process from material having very low coefficients of thermal expansion (10.times.10.sup.-7 /.degree.C. or less over the range of 0.degree. to 1000.degree. Centigrade). The wheels were formed having cement bond joints extending continuously across the open annular faces of the resulting wheel. In attempting to construct industrial sized heat recovery wheels (approximately 28 inches (71 cm) or greater in diameter) from materials having greater coefficients of thermal expansion (approximately 18.times.10.sup.-7 /.degree.C. or more over the range 0.degree. to 1000.degree. Centigrade), it has been learned that this prior method of joining the cellular segments to one another is a significant cause of thermally induced stresses in the joint areas of the wheel during operation. Some thermal stress reduction in these areas can be achieved by creating discontinuities through the joints in the direction extending in the direction of the fluid flow through the honeycombed structure and is the subject of a co-pending application Ser. No. 205,775 filed Nov. 10, 1980 and assigned to the assignee of this application.
Heat recovery wheels are typically operated in counterflow heat exchanger systems. The wheels are rotated through opposing flows of relatively hot and cold gases, the hot gas heating the matrix material when passing through it and the cold gas absorbing the heat held by the matrix material while passing through the wheel during the second half of its rotation. By passing the gases in opposite directions through the wheel, one annular face of the wheel can be maintained at a higher average temperature than the other, increasing the thermal efficiency of a given wheel and reducing the thermal shock to which each face is subject. Applicants have determined that the previously employed method of bonding cellular segments together by using continuous joints which extend up to and across the open annular faces of the wheel was a significant cause of spalling and fracture of the matrix, especially on the face of the wheel exposed to the highest temperatures, due to nonlinear temperature differences which develop in the axial direction in the matrix and the tendency of the bonding material to transmit stresses between the cellular segments. Applicants believe that these relationships were not heretofore perceived.